Life on earth appears to be surrounded by O2. It is thus surprising to the uninitiated that there exist many microorganisms, both prokaryotic and eukaryotic, whose growth and survival require the complete absence of O2. Despite its abundance, O2 is actually depleted in many places, such as sediments rich in rotting organic matters and the alimentary tract of humans and animals. Anaerobic organisms (anaerobes) thrive in these O2-free environments. They not only play a major role in the cycling of carbon, nitrogen, and sulfur on earth but have a profound effect on the health of humans and animals.

Because anaerobes cannot use O2, the terminal electron acceptor, they are required to use a variety of other inorganic or organic substances as an electron sink to balance the oxidation-reduction reactions. The reduced end products (such as H2, CH4, and alcohols) are energy-rich, and some can accumulate to a high level to make the producing organism commercially important. To be functional, enzymes of anaerobes must cope with some of the intracellular conditions that are characteristic of anaerobic growth, such as a high concentration of alcohols. Some structural features may differentiate homologous proteins from aerobes and anaerobes, and these proteins are useful for the study of structure-function relationships.

Research in my laboratory is focused on the metabolic activities that give an anaerobe a potentially useful trait, which may serve as the basis for an industrial application or as the target of an antimicrobial agent. We study the molecular properties of enzymes, their structural genes, and the control elements for these genes. We also study gross properties of anaerobes as related to the molecular characterization. Currently, my laboratory is conducting research in two areas: solvent production and nitrogen fixation in the clostridia.

Solvent production
Several Clostridium species produce a commercially important amount of acetone, butanol, and isopropanol (solvents), which are important industrial chemicals and fuel additives. Besides being an established industrial process, solvent production by clostridia provides a system for the study of (1) regulation of multiple-branched pathways, and (2) the expression of genes subjected to control by environmental signals. We have purified and characterized most solvent-forming enzymes from C. beijerinckii, and their structural genes have been cloned and sequenced. The study revealed an array of alcohol dehydrogenases (ADHs) in these clostridia. Properties of one novel ADH were successfully used to manipulate and increase solvent production, and the structural gene for this ADH is now used as a reporter to study gene expression in these clostridia. The solvent-production genes are tightly regulated. A focus of our current research is to determine the control mechanism for the expression of these genes.

Nitrogen fixation
Nitrogen fixation is catalyzed by a structurally conserved enzyme nitrogenase, which is sensitive to inhibition by H2. We would like to learn how nitrogenase of C. pasteurianum is able to maintain its activity when the cell is producing a copious amount of H2 during growth. C. pasteurianum belongs to a phylogenetically distinct group that diverged from other bacteria early. The nitrogen-fixation (nif) genes of C. pasteurianum have unusual structural features and seem to be more functionally grouped than in other organisms. The promoters for the clostridial nif genes are also unusual. Recently, the genome sequence for the solvent-producing Clostridium acetobutylicum became available, and it contains a nif cluster similar to what was found in C. pasterianum. We are studying the distinct nif genes and their products in the clostridia to gain insight into the function and synthesis of nitrogenase.

Kasap, M. and Chen, J.-S. (2005) Clostridium pasteurianum W5 synthesizes two NifH-related polypeptides under nitrogen-fixing conditions. Microbiology, 151:2353-2362.   [Abstract]

Chen, J.-S., Toth, J., and Kasap, M. (2001) Nitrogen-fixation genes and nitrogenase activity in Clostridium acetobutylicum and Clostridium beijerinckii. J. Indust. Microbiol. & Biotechnology 27:281-286.   [Abstract]

Toth, J., Ismaiel, A. A., and Chen, J.-S. (1999) The ald gene, encoding a coenzyme A-acylating aldehyde dehydrogenase, distinguishes Clostridium beijerinckii and two other solvent-producing clostridia from Clostridium acetobuylicum. Appl. Environ. Microbiol. 65:4973-4980.   [Abstract]

Peretz, M., Bogin, O., Tel-Or, S., Cohen, A., Burstein, Y., Li, G., and Chen, J.-S. (1997) Molecular cloning, nucleotide sequencing, and expression of genes encoding alcohol dehydrogenases from the thermophile Thermoanaerobacter brockii and the mesophile Clostridium beijerinckii. Anaerobe 3:259-270.

Johnson, J. L., Toth, J., Santiwatanakul, S., and Chen, J.-S. (1997) Cultures of "Clostridium acetobutylicum" from various collections comprise Clostridium acetobutylicum, Clostridum beijerinckii, and two other distinct types based on DNA-DNA reassociation. Int. J. Syst. Bacteriol. 47:420-424.   [Abstract]

Johnson, J. L. and Chen, J.-S. (1995) Taxonomic relationships among strains of Clostridium acetobutylicum and other phenotyptically similar organisms. FEMS Microbiol. Rev. 17:233-240.

Chen, J.-S. (1995) Alcohol dehydrogenase: Multiplicity and relatedness in solvent-producing clostridia. FEMS Microbiol. Rev. 17: 263-273.   [Abstract]